1. Field of the Invention
The present invention concerns the general technical field of the metering of the flow of a fluid circulating in a circuit by means of a pulse response flowmeter and it concerns, more precisely, the calibration of such flowmeters.
2. Description of the Relate Art
The invention is particularly advantageous in metering the flow of a dialysis liquid circulating in a haemodialyser of an artificial kidney.
In the above mentioned preferred application, an artificial kidney comprises in the conventional way a haemodialyser with two compartments separated by a membrane, one compartment being connected to the circuit of the dialysis liquid, and the other compartment being connected to a patient via an extracorporeal blood circuit. The circuit of the dialysis liquid is provided upstream and downstream of the haemodialyser, with flowering generating an electric pulse as a specified fraction of liquid passes.
During a dialysis session, the excess liquid present in the blood is subjected to ultrafiltration through a membrane because of the pressure gradient on either side of the membrane. Before such a session one proceeds with a stage of calibrating the flowmeters for which the haemodialyser is bypassed and an identical flow of dialysis liquid circulates in the two flowmeters. This calibration stage includes metering the calibration frequencies of each flowmeter by counting the number of pulses during a chosen period, to define the relation existing between a metering frequency and a given rate of flow of the dialysis liquid. Such a calibration stage permits a partial correction of the intrinsic errors of the flowmeters, and also permits the use flowmeters of different types.
The calibration of the flowmeters deriving from experimental measurements is vitiated by a certain error systematically affecting the results of the subsequent measurements. It is therefore indispensable to reduce as far as possible the accidental metering errors liable to occur during the calibration stage, to obtain a maximum metering accuracy for determining the ultrafiltrate removed from the patient's blood.
To attempt to reduce the importance of the metering errors, such as artifacts, the prior art generally proposes to proceed with a calibration step of a rather long duration.
Moreover, the patent application EP A 88 304 162.6 (publication no. 0 298 587 A2) proposes a method tending to limit the errors in flow measurements, in particular during a calibration stage of the flowmeters equipping a haemodialyser. This document proposes to mount in series in the dialysis liquid circuit a pair of flowmeters upstream of the haemodialyser and another pair downstream of the haemodialyser. During the calibration stage, the maemodialyser is bypasses so that the four flowmeters are placed in series. The frequencies of the flowmeters are measured and the calibration is considered to be valid if the deviations between the measured frequencies fall within a specified range of values.
The main drawback of the method described above lies in the fact that the accuracy of the metering depends directly on the acquisition period during which the pulses are counted. Thus in order to reach a high accuracy, it is necessary to increase the time during which the measurements are acquired to a prohibitive extent.
Moreover, it has proved in practice, that the flowmeters show a metering disparity during a dialysis session generally lasting four hours. It then becomes necessary to proceed with calibrations during the dialysis session to eliminate this error, which reduces the dialysis efficiency because of the long duration of the calibration stage.
Moreover, the doubling of the flowmeters increases the operating cost of such a method.